Hi,
What is different between:

1) OR V.S. ||
2) $_X V.S. $X
3) @session_start(); V.S. session_start();
4) if(!$X == Y) V.S. if($X != $Y)
5) if($X == $Y) V.S. if($X === $Y)
6) if($X == '') V.S. if($X == null)
7) die(''); V.S. exit();

?
Regards.

Each of these require a little more than one line to adequately explain, so I will cherry pick #4.

As you know, a conditional statement, such as the if block, will only be entered if the value is true. So by prepending an exclamation mark to a variable, you are inverting its value. However, many values cannot be successfully inverted. A string, for instance, cannot be inverted -- only reversed. To an invert a word, I would assume, you would have to find its antonym. In computer languages, such as PHP, this does not apply, as everything is reduced to either true or false, 1 or 0, yes or no.

Thus, if I inverted a string it would naturally be inverted to false. Conversely, if I were to invert a false value, it would become true. Therefore once you place that in a conditional if statement, any variable that is true, will become false, and thus not enter that if statement.

For instance:


To enter the if statement block, the initial value would have had to have been false to begin with, or any value which denotes a false value -- such as null.

Why did I write all that? Well, I personally think that's the best way to understand your #4 question. Both are somewhat similar, but in the first example you're inverting the value, and in the second you're merely looking for a non-matching value. Both, insofar as I can see, will produce the same results, but the latter should be used, at least in my opinion, in such an instance. I reserve the inverting of values for if statements, as seen above.

In addendum, you can invert values as much as you like. Consider the following:


Inverted the variable 3 times, from its initial state of false:

• True
• False
• True (Final state)

I hope this answers your question thoroughly!

__________________
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.

Personal preference, they are identical.
The _ just changes the variables name, there's nothing special about it.
It's like comparing $a to $b .
However, some superglobal variables (i.e. $_SESSION, $_GET, $_POST) have an underscore in them and I presume it is a safegaurd so that users don't accidentally overwrite / change variables.
The @ suppresses any errors that the session_start() function might cause. Take a look at PHP: Error Control Operators - Manual for more info.
I think Wildhoney's covered this pretty well, so I won't say too much.
I'm pretty sure that the first statement means - If the inverse of $X is equal to $Y , and the second one means if $X is not equal to $Y
Read Wildhoney's post
First checks if $X evaluates to an empty string. The second checks to see if $X evaluates to Null. Also read wildhoney's post
I'm pretty sure they are identical.

I know of at least one instance where the #6 differs. Take the following as an example:


Juxtaposed with:


I surmise that's something to do with the fact that the array is null. It is not null to the variable because it holds a skeleton of an array, but the array, per se, is null.

When you attempt to validate it by specifying a string value, I.E: '', then you are attempting to classify it as a string, and as aforementioned, the variable is not null in that sense because it holds an array -- an empty, or null, array.

Am I right, or am I way off? Somebody please help.

__________________
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.

Thanks both ;). Thats resolved!

Wrong, they operate under different precedence rules.

PHP: Logical Operators - Manual

PHP logical operator precedence:
On the face of it, it seems like both operate the same thus both evaluate to 'true' ('or' and '||' gives us 'true' if either the expression result to the left or right is true) however
this wont behave as expected, and $f will be assigned 'false', why?

Well if you look at the precedence chart on php.net, you see that 'or' is lower down the list than '||'
this means that it has lower precedence, this isn't the issue however. The issue arises when you use it in conjunction with another operator, for example '='
this is higher in the list than 'or', so therefore any expression with '=' in it must be evaluated first, thus:( i will use brackets to show what PHP does first)
So there you can see, PHP creates $f and assigns it a boolean 'false' before the 'or' statement is reached, and because assigning 'false' to $f didn't produce 'false' i.e. it didn't fail
it exits the expression there, leaving $f equal to 'false'.

Looking at the other example:
'||' is higher than 'or' and more importantly '=', therefore the expression functions like expected:
Which gives us 'true' because the '||' expression is evaluated before '=' assigns which gives us 'true' as expected.

Hopefully that explains it correctly.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)

Thanks for explaining that, I didn't realise that realise that operators other than the numerical ones had an order of precedence.

np, it is an easy one to miss as on the surface they seem to operate identically and for the most part do so, plus the php documentation really should be clearer.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)